From a corrosion point of view, the presence of dissolved gases, even in small amounts, is undesirable in water systems which contact metal surfaces. For example, metal surfaces in contact with oxygen-containing industrial water can experience severe pitting. Pitting is highly concentrated corrosion affecting only a small area of the total metal surfaces. This can, however, be a serious problem causing metal failure even though only a small amount of metal is lost and the overall corrosion rate is relatively low.
With respect to oxygen, the severity of attack will depend on the concentration of dissolved oxygen in the water, water pH and temperature. As water temperature increases, as for example in a water heating system, enough driving force is added to the corrosion reaction that small amounts of dissolved oxygen in the water can cause serious problems. Oxygen pitting is considered to be a most serious problem in boiler systems, even where only trace amounts of oxygen are present.
Deaeration is a widely used method for removing oxygen from an oxygen-containing aqueous medium. It is particularly useful for treating boiler feedwater and can be either mechanical or chemical.
While vacuum deaeration has proven to be a useful mechanical deaeration method for treating water distributing systems, boiler feedwater is treated using pressure deaeration with steam as the purge gas. According to the pressure deaeration method for preparing boiler feedwater, the water is sprayed into a steam atmosphere and is heated to a temperature at which the solubility of oxygen in the water is low. About 90 to 95 percent of the oxygen in the feedwater is released to the steam and is purged from the system by venting.
Mechanical deaeration is considered an important first step in removing dissolved oxygen from boiler feedwater. However, as already noted, as water temperature increases, even trace amounts of dissolved oxygen can cause serious problems. Accordingly, supplemental chemical deaeration is often required.
For boilers below 1000 pounds per square inch (psi), catalyzed sodium sulfite is used as an oxygen scavenger for the chemical deaeration of the feedwater. This chemical's oxygen scavenging property is illustrated by the reaction: ##STR2## The oxygen-sulfite reaction can be effectively catalyzed by iron, copper, cobalt, nickel and/or manganese. While the sodium sulfite oxygen scavenger is often used with success, this material still has its limitations. At boiler operating pressures of 900 to 1000 psi and above, increased dissolved solids from the sulfite-oxygen reaction product can become a significant problem. Also, at high pressures the sulfite decomposes in the boiler to form sulfur dioxide and hydrogen sulfide, both of which can cause corrosion in the return condensate system.
Hydrazine is also used as an oxygen scavenger according to the reaction: ##STR3## This chemical does not have the above-noted high pressure limitations of sodium sulfite. For example, since the products of the hydrazine-oxygen reaction are water and nitrogen, no solids are added to the boiler water. Hydrazine as an oxygen scavenger does, however, have its own limitations. A major problem relates to its extreme toxicity and carcinogenicity. Also, the hydrazine-oxygen reaction is very slow at low temperatures which might be encountered such as at room temperature. Above 400.degree. F., hydrazine also decomposes according to: ##STR4## The ammonia decomposition product can be aggressive to copper or the copper-bearing metallurgies that are found in condensate systems.
In an attempt to obviate some of the problems encountered by the use of hydrazine, it has been proposed to combine hydrazine with certain accelerator or catalyst compounds. For instance, U.S. Pat. Nos. 3,728,281 (Marko et al); 3,843,547 (Kaufman et al); 3,983,048 (Schiessl); and 4,124,500 (Arghiropoulos et al) all disclose that various aryl amine compounds may be added to the hydrazine solution. Other hydrazine additives are noted in U.S. Pat. Nos. 3,551,349 (Kallfass); and 4,012,195 (Noack).
Despite the efforts of the prior art, there remains a need for an oxygen scavenger or scavenger system which eliminates the use of hydrazine altogether while still effectively acting to remove or reduce dissolved oxygen from the desired aqueous medium.